If you’ve noticed recent damage to the current-year needles on your Douglas-fir, you are not alone. The damage looks similar to that of western spruce budworm or Rhabdocline needle cast, but as it turns out, much of the damage is being caused by the Douglas-fir needle midge.
The Douglas-fir needle midge is a tiny fly (5 mm long) that has one generation a year. The adults emerge in the spring and can be seen resting on the tips of needles. The females deposit eggs in groups on newly expanding buds. The eggs hatch in a few days and the larvae bore into the needles and feed on them throughout the summer. In response to the feeding, galls form in the needles causing them to appear bent and distorted. Eventually the needles become discolored and finally drop from the tree in fall and winter.
The midge larvae drop from the needles to the ground in the fall as well, and then overwinter as larvae in the soil. In the spring, the larvae pupate in the soil and the adults emerge, completing the life cycle. Emergence typically occurs between April and May, depending on weather conditions (rain and cold temperatures delay emergence).
Infestation can be as high as 100 percent of the needles on a tree. It may take several years for trees to recover as a result of the ensuing needle loss. If trees are defoliated for several consecutive years, twig dieback can occur. Generally, damage caused by the midge is not a mortality-inducing factor.
Management of the Douglas-fir needle midge is usually not necessary or economically feasible in a forested setting. Depending upon the severity, management is sometimes necessary in Christmas tree plantations or seed tree orchards.
Typical management of the midge involves a trapping period followed by application of insecticide. Because timing of insecticide application is critical (within a week of adult emergence), traps must be used to ensure proper timing. Late application of chemicals results in little or no control and can worsen midge problems by killing the later-emerging parasitoids, which act as the midges’ natural control agents. If you feel that insecticide application may be warranted on your property, contact your local extension agent for more information.
For further reading:
DeAngelis, J.D. 1994. Biology and control of Douglas-fir needle midge in Christmas trees. Oregon State University Publication EC 1373-E. 2 pp.
Douglas-fir needle midge Contarinia pseudotsugae Condrashoff. Penn State University.